Dear R-users,

I  am checking the proportional hazard assumption of a cox model  for a
given covariate, let say Z1, after adjusting for other relavent covariates
in the model. To this end, I fitted cox model stratified on the discrete
values of Z1 and try to get beslow estimator for the baseline cumulative
hazard function (H(t)) in each stratum. As far as i know, if the
proportionality assumption holds,  the  plot of ln[H(t)] of each stratum
versus time should be approximately parallel.

i.e
fit<-coxph(Surv(start,end,status)~sx+rated+AGLEVEL+strata(Z1),data=ALLDPinfectionandbronchitis)
ss<-survfit(fit)
plot(ss,fun="cumhaz")

My question is on whether the cumulative hazard given by the above command
is actually a breslow estimator for baseline cumulative hazard ,i.e,
estimator=sum(  number of death/ (sum(risk score in risk set))   or  a
nelson-Aalen estimator. if the above command does not give me breslow
estimator, please advise on how I can get it.

Thanks for any help given.

kind regards,
sing yee

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